Abstract/Summary

The Forres area was subjected to one of the most catastrophic floods in UK history when the River Findhorn flooded in 1829. In recent times, Forres has also flooded (notably in 1997 and 2001) but to a much lesser extent. To help protect the town of Forres a flood alleviation scheme against flooding from the River Findhorn is proposed for west Forres and the Pilmuir area. This scheme involves a series of embankments and river channel engineering to stop the eastward flow of water through Forres, and an open drainage channel to drain storm water in the Pilmuir sub catchment. A critical issue in the operation and effectiveness of the flood alleviation scheme is the role of groundwater.
Groundwater investigations and modelling in the Pilmuir area of Forres were first undertaken by BGS from Jan – May 2007 (reported in MacDonald et al. 2007). The investigations
showed that the superficial deposits are highly permeable, contain much groundwater, and could therefore impact the success of the flood alleviation measures. Therefore, a second phase of work was proposed which examined the groundwater conditions around the area of the proposed embankments and modelled the effect of various flooding scenarios on general groundwater conditions and the flows in the drains/channels. BGS were commissioned to carry out this work during a study from November 07 to March 08. The following work was undertaken.
1. The hydrogeology of the area to be inundated was characterised by undertaking 13 short pumping tests in newly drilled piezometers, supervising 7 others and analysing and interpreting results. Groundwater-level data from installed divers were also interpreted.
2. Groundwater samples for chemistry analysis and residence time indicators were also taken at 13 sites and the samples analysed and interpreted.
3. Topsoil permeability was measured in 33 locations by the Macaulay Institute using a Guelph Permeameter.
4. Using the new data, the groundwater flow model initially developed in Phase 1 was modified and extended to help test the effect on general groundwater levels and drain/channel flows of storing floodwater on the floodplain behind the bund and explore the influence of a cut-off wall in the vicinity of the garden centre.
5. Using existing rainfall data, hydrogeological understanding and the groundwater model, a worst case groundwater flooding scenario was estimated.